Two-fermion system wave function

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Discussion Overview

The discussion revolves around the wave function of a two-fermion system, specifically focusing on two-electron atoms and their representation in terms of symmetric and antisymmetric functions. Participants explore the implications of energy levels, superpositions, and the conditions under which different wave function forms are applicable.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant notes that the overall wave function for a two-electron atom can be either a symmetric spatial function with an antisymmetric spin function or an antisymmetric spatial function with a symmetric spin function, questioning why a superposition (a + b) is not used in this context.
  • Another participant suggests that if the two-electron atom does not have a superposition of energy levels, the spins and spatial components must be fixed as either symmetric or antisymmetric, but not both, implying that superpositions lead to the possibility of using a + b.
  • A participant points out that in the first excited state of the atom, where one electron is in the lowest energy level and the other in a higher level, the expected wave function does not include a superposition, only a term from the initial post.
  • Further clarification is provided that fixing the energy levels means there is no superposition of different energy states, contrasting this with scenarios like Rabi oscillations where electrons may not be in specific energy levels consistently.
  • Another participant raises a question about determining whether to use the symmetric or antisymmetric wave function when the electrons have different energy levels, noting that for identical energy levels, the antisymmetric spatial wave function becomes zero.
  • One participant mentions that while the antisymmetric spatial wave function is often preferred due to resulting in larger mean separations of electrons, both forms are valid and the choice depends on specific circumstances, citing exceptions like the singlet ground states of most molecules versus the case of oxygen2.

Areas of Agreement / Disagreement

Participants express differing views on the conditions under which to use symmetric or antisymmetric wave functions, particularly regarding the role of superpositions and energy levels. The discussion remains unresolved with multiple competing perspectives on the topic.

Contextual Notes

Limitations include the dependence on specific definitions of energy states and the nature of superpositions, which are not fully explored in the discussion.

PhyPsy
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For a two-electron atom, this book says that the overall wave function is either a) the symmetric space function times the antisymmetric spin function or b) the antisymmetric space function times the symmetric spin function. However, in another problem which involves two fermions in a harmonic oscillator, it says the overall wave function is a sum of the two aforementioned products (a + b). Why would the atom's wave function not be a + b instead of being just a or b?
 
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If your two-electron atom does not have a superposition of different energy levels for the electrons (something you have to carefully prepare), the spins are fixed and antisymmetric OR symmetric, but not both, and the same is true for the spatial component. If you have superpositions of different energy levels, you might get something like "a+b".
 
For the first excited state of the atom, one electron is in the lowest energy level n=1, and the other is in the level n=2. According to your statement, there should be a superposition, but the answer only contains a b (from my initial post) term and no a term.
 
PhyPsy said:
For the first excited state of the atom, one electron is in the lowest energy level n=1, and the other is in the level n=2.
If you fix the energy levels like this, you do not have a superposition of different energy states.

What I meant is something like Rabi oscillations, where the electrons are not in specific energy levels (at least not all the time).
 
OK, so then for systems that are not a superposition, how do you know whether to use a or b? I understand that for atoms where both electrons have the same value for n, the antisymmetric space function is equal to 0, so you have to pick a, but what about atoms where the electrons have different values for n? Is it always b for atoms with electrons that have different values for n?
 
In the case of electrons, usually the antisymmetric spatial wave function is preferrable, since this leads to a larger mean separation of the electrons (look up the Hund rules).

But both are valid wave functions, and which one is preferable depends on the circumstances. For example, most molecules have singlet ground states, but then there is oxygen2, which does not.
 

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